Linux File Permissions: The Commands You'll Use Daily

Misconfigured permissions cause problems constantly. SSH refuses your key. A script won't execute. Config files are world-readable when they contain secrets.

Here's what you need to know.

Reading Permissions

ls -l myfile.txt
-rw-r--r-- 1 user group 1234 Jun 20 12:34 myfile.txt

That -rw-r--r-- breaks down as:

• - = regular file (d = directory)
• rw- = owner can read and write
• r-- = group can read
• r-- = others can read

Three permission types:

• r (read) = view file contents
• w (write) = modify file
• x (execute) = run as program, or enter directory

Octal Notation

Once you learn the numbers, it's faster:

Octal	Permissions
7	rwx
6	rw-
5	r-x
4	r--
0	---

So chmod 755 script.sh means:

• Owner: 7 (rwx)
• Group: 5 (r-x)
• Others: 5 (r-x)

Common Permission Patterns

Scripts (755):

chmod 755 backup.sh

Owner can edit, everyone can run.

Config files with secrets (600):

chmod 600 database.yml

Only owner can read. This is what you want for files containing credentials.

SSH private keys (600 or 400):

chmod 600 ~/.ssh/id_rsa
chmod 400 ~/.ssh/aws-key.pem

SSH refuses to use keys with looser permissions. AWS recommends 400 (read-only for owner).

Shared directories (770):

chmod 770 /project

Owner and group have full access, others have none.

Changing Permissions

Octal notation:

chmod 644 file.txt

Symbolic notation:

chmod u+x script.sh     # Add execute for owner
chmod g-w file.txt      # Remove write from group
chmod o-rwx secrets.txt # Remove all from others

Changing Ownership

chown user:group file.txt
chown -R user:group directory/

The -R flag applies recursively.

Common Problems

"Permission denied" on script:

chmod +x script.sh

Scripts need execute permission.

SSH key rejected:

chmod 600 ~/.ssh/id_rsa

SSH enforces strict permissions on private keys.

Can't cd into directory: Directories need execute permission for traversal, not just read:

chmod 755 directory/

Finding Permission Problems

Find world-writable files:

find /path -type f -perm -o+w

Find files owned by specific user:

find /path -user username

Key Takeaways

• Master octal: 755 for scripts, 644 for files, 600 for secrets
• SSH keys must be 600 or stricter - SSH enforces this
• Directories need execute (x) permission for users to enter them
• Use 600 for any file containing credentials
• Start restrictive and loosen only when needed
• find with permission flags helps audit configurations